1. Field of the Invention
The present invention relates to a method and an apparatus for resolving a reactive gas and depositing a film on a substrate by chemical vapor deposition (CVD) using a laser beam, and more particularly to a film formation method and apparatus which make it possible to control the concentration of the reactive gas with high precision and to change the concentration in a short period of time.
2. Description of the Related Art
Film formation methods using laser CVD are generally greater in the velocity at which the film is deposited on the substrate, in other words the reaction rate of the reactive gas, than film formation methods using reduced pressure CVD, plasma CVD or metal organic CVD (MOCVD) which are applied to semiconductor manufacturing processes. Accordingly, in the methods using laser CVD, the volume, namely the concentration, of the reactive gas supplied into the chember significantly affects the stability of the film formation process and the properties and shape of the film that is deposited. In view of this fact, the requirement to accurately control the supply volume on the reactive gas has become increasingly stringent in recent years so that the film can be formed exactly as intended.
In a laser CVD apparatus according to the prior art disclosed in the Japanese Patent Laid-open No. 1992-295851, a reactive gas 2 is supplied into a chamber 3 by a gas supply source 1 as illustrated in FIG. 1. In a reservoir 4, constituting the gas supply source 1, organic metallic compound 5, such as chromium carbonyl (Cr(CO).sub.6) powder, is stored in advance. By heating the reservoir 4 to a prescribed temperature, the organic metallic compound 5 is sublimated to obtain the reactive gas 2. A predetermined amount of one kind of carrier gas 6, such as argon (Ar) gas, is fed into the reservoir 4, and reactive gas 2 of a desired concentration is obtained by mixing this carrier gas 6 and reactive gas 2. That reactive gas 2 adjusted in concentration is supplied into the chamber 3, wherein the laser CVD process takes place. The concentration of the reactive gas 2 in this conventional laser CVD apparatus is managed by controlling the flow rate of the carrier gas 6, fed into the reservoir 4 whose temperature is kept constant. The flow rate of the carrier gas 6 is controlled by a flow rate regulator, such as a mass flow controller 7.
Incidentally, in the chember 3 as illustrated in FIG. 1, a substrate 8 is mounted on an XY stage 10, and is irradiated with a laser beam 12 via a glass window 11.
Generally, the way the reactive gas 2, supplied from a gas inlet port 9, flows differs between the case shown in FIG. 2A wherein the film is deposited on the substrate 8 in its central part and that shown in FIG. 2B wherein the deposition takes place on the peripheral part of the substrate 8. Especially in the peripheral part of the substrate 8, where the leak of the reactive gas 2 from the substrate 8 is present in a large amount, the concentration of the reactive gas 2 becomes diluted. Therefore, when the area in which the film is to be deposited is the peripheral part of the substrate 8, the concentration of the reactive gas 2 introduced from the gas inlet port 9 has to be increased in order to deposit a film of high quality. Especially, when a plurality of films are to be deposited on the same substrate 8 in its central and peripheral parts, the concentration of the reactive gas 2 has to be changed quickly to match each particular area of deposition.